“At this time we are responding to a fire, not a chemical release,” the Texas Commission on Environmental Quality said in a statement. “Our focus remains on the safety of those around the facility, and we urge everyone in the area to follow the safety instruction of local authorities, specifically avoiding smoke and floodwaters.”

Black smoke from a fire still burning at the Arkema plant Wednesday is dangerous, said Terry Collins, a Teresa Heinz professor of green chemistry at Carnegie Mellon University in Pittsburgh: “You do not want to be breathing fumes from this plant.” The Texas Commission on Environmental Quality and the Environmental Protection Agency evacuated 300 homes around the facility in Crosby, Tex., in anticipation of the fire days before it happened.

Fifteen sheriff’s deputies and first responders were rushed to a hospital as a precaution and decontaminated after complaining of irritation and itching from smoke that wafted from the plant. They were released, and a sheriff’s official said, “They’re all fine.”

Thousands of pounds of unstable chemicals known as organic peroxides were stored in 10 18-wheel trailers that used fans to keep the chemicals stable. One trailer ignited when the fans failed, a process that company officials expect will repeat at least eight times as fans in other trailers stop operating.

Organic peroxides are harmful when they come in contact with the eyes, lungs or skin. But water dilutes the chemicals, and humidity can ease their effect when they become airborne. No injuries related to the chemical have been reported, though some residents in Houston said they smelled an odor Thursday that wasn’t there the previous day.

State environment officials urged residents near the plant to limit their exposure to smoke and remain indoors with their doors and windows closed. “If it is absolutely necessary to be outdoors, try to move out of the plume of smoke and minimize heavy work, exercise or children’s playtime,” agency officials said.

But flooding around numerous petrochemical plants worried experts who understand the threat of chemicals housed within. “As both a Texan and a toxicologist, I am extremely worried about the consequences of the hurricane in a coastal industrial area,” said Andrea C. Gore, the Vacek chair of pharmacology at the University of Texas at Austin. “The volume of stored chemicals is enormous, and considering the destruction, I wonder just how much has gotten into the environment.”

As part of an investigative report on the coastal chemistry industry last year, the Houston Chronicle included an analysis by the Mary Kay O’Connor Process Safety Center at Texas A&M University that ranked companies in terms of their potential public harm. Arkema was ranked 21st out of more than 50 companies.

A majority of the Arkema plant sits in a 100-year flood plain and is deluged by flooding. Organic peroxides are highly flammable, “as was obvious by the explosion,” Gore said. When the compound comes into contact with other substances, they make those less stable, as well.

“This can result in further explosions and unanticipated chemical reactions. Destabilized chemicals are more likely to leach or get released into the environment, since their storage containers were designed for the original chemical, not its destabilized by-products,” Gore said. “So my question is: With what other chemicals and materials in Arkema did the massive release of organic peroxides come into contact?”

The Arkema chemical plant in Crosby, Tex., is flooded from Harvey. The plant lost power and its backup generators, leaving it without refrigeration for chemicals that become volatile as the temperature rises. (Godofredo A. Vasquez/Houston Chronicle via AP)

Because hundreds of chemicals qualify as organic peroxides, it’s hard to make a blanket statement about the risks they pose to people who could be exposed. “Those health impacts can vary based on a lot of factors,” including how exposure happened and its duration, said Patrick Krieger, assistant director for regulatory and technical affairs at the Plastics Industry Association.

Organic peroxides are a broad family, and their volatility makes them essential in manufacturing. “A lot of people don’t really realize what they are. They are so important to the modern manufacturing and industrial manufacturing process,” he said, noting that they are used to produce pharmaceuticals, skin-care products, adhesives, disinfectants and a wide array of plastics, among other things. “They are very important to our modern lives.”

But the same reaction that makes organic peroxides useful in helping manufacture plastics also makes them dangerous if handled carelessly, according to the association’s website. “Following recommended storage and handling practices can effectively reduce the risk of fires or explosions associated with organic peroxides,” it says.

Some experts question whether a company that houses chemicals as unstable as organic peroxides should be located in a metropolitan area that floods as often as Houston.

But Collins, the Carnegie Mellon expert, said Arkema appears to be taking the right precautions to contain a potentially harmful product. “Arkema is a big company making various kinds of peroxides throughout the world,” he said. “These kinds of compounds don’t let you make mistakes. They’ll let you know you’re not being safe. My guess is they’re very careful.”